There is a known bedded track in which a track bed is provided on a roadbed, and sleepers and rails are disposed on the track bed. Such a bedded track typically has a structure such that sleepers are disposed on the track bed formed on the roadbed, and a pair of rails are fastened to the sleepers so as to be spaced at a certain distance and parallel to each other. The track bed constituting the bedded track are made of ballast, slab, and the like, which is selected considering various conditions of each railway. A ballast bed track 1000 using ballast 2, such as gravel, crushed stones, or the like, as exemplarily shown in FIG. 5A and FIG. 5B, has been most known among them. A track bed 1003 using the ballast 2 has been employed for many years since the track bed 1003 supports running of heavy railway cars in a rational manner and is also economically competitive due to the nature thereof. The ballast 2, such as gravel, crushed stones, or the like, has functions to firmly support sleepers 4, to evenly distribute a load transmitted from a train through rails 5 and the sleepers 4 over the roadbed, and to give elasticity to a track as well as to facilitate maintenance work, such as tamping, and to give good drainage of the track thereby to prevent mud-pumping or weed incidence.
However, in the track bed using the ballast as described above, individual gravel particles or crushed stones tend to move in a vicinity of a surface of the track bed. The track bed is easily deformed due to passing train loads, a bulging force resulting from a rail axial force at a high temperature, stress by an earthquake or the like, or subsidence of an embankment structure, and thereby a height deviation of the track is sometimes caused. Accordingly, when such a height deviation of the track is caused, correction is made by refilling the ballast or tamping the track bed. Such an operation to correct the height deviation of the track requires considerable work and cost.
There is also a known track bed using ballast (see, for example, Patent Document 1) in which rigid plates are provided between a plurality of sleepers on the ballast and the rigid plates are mutually connected by upwardly U-shaped anchors passing through under the sleepers. However, in an area from a toe of slope to a top of slope of the track bed, individual gravel particles or crushed stones tend to move in a vicinity of a surface of the track bed as described above, and the track bed is easily deformed due to passing train loads, a bulging force resulting from a rail axial force at a high temperature, stress by an earthquake or the like, or subsidence of an embankment structure, and a height deviation of the track is sometimes caused.
Therefore, as illustrated by an example in FIG. 5C and FIG. 5D, there has been devised a ballast bed track 1100 (see, for example, Patent Document 2) in which ballast retaining structures 1010 are provided on both sides of a track bed 1103 on a roadbed 1 so that the ballast retaining structures 1010 can suppress deformation of a ballast 2. The ballast retaining structures 1010 which are made of concrete, such as prestressed concrete, are formed in a wall shape.
However, the above-described ballast retaining structure 1010 per unit which is formed in a wall shape with, for example, a width of 50 cm weighs approximately 200 kg. This leads to problems in construction work as follows: (1) It is required to transport the ballast retaining structures by a maintenance car or the like from a storage area to a construction cite at the time of construction. (2) It is required to use heavy equipment for construction. (3) In a case where the ballast retaining structure 1010 has a projection 1010a, it is required to bury the projection 1010a in the roadbed 1 and thus it is required to widely dig the track bed in a cross-sectional direction perpendicular to rails and backfill the track bed after disposing the ballast retaining structure 1010. Also, it is known that a construction method using the ballast retaining structure 1010 generally results in higher costs.
Then, there has been devised a method in which flexible cloth or polyethylene sandbags filled with a filling material, such as gravel or crushed stones, are disposed on a roadbed, surfaces of the sandbags are covered with crushed stones or the like to form a flat plane, sleepers are disposed on the flat plane, and rails are fastened to the sleepers (see, for example, Patent Documents 3-6). According to the method using sandbags filled with a filling material, subsidence of the rails due to repeated loads by passing railway cars can be reduced and thereby maintenance work for the rails and the track bed can be reduced, and also vibration and noise during the passing of the railway cars can be reduced, compared with the conventional method of simply laying gravel or crushed stones.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 9-111704    Patent Document 2: Japanese Unexamined Patent Application Publication No. 8-144206    Patent Document 3: Japanese Unexamined Patent Application Publication No. 8-151601    Patent Document 4: Japanese Unexamined Patent Application Publication No. 8-74201    Patent Document 5: Japanese Unexamined Patent Application Publication No. 2000-86890    Patent Document 6: Japanese Unexamined Patent Application Publication No. 2001-271301    Patent Document 7: Japanese Unexamined Patent Application Publication No. 8-27701    Patent Document 8: Japanese Unexamined Patent Application Publication No. 9-137422